The present invention relates to fluid coupling devices, and more particularly, to such devices which are capable of changing between the engaged and disengaged conditions, in response to variations in a predetermined temperature condition.
A fluid coupling device of the type to which the present invention relates typically includes an input coupling member and an output coupling member which cooperate with each other to define a viscous shear space such that torque may be transmitted from the input member to the output member in the presence of a viscous shear fluid. The output coupling member typically defines a fluid chamber and a valve plate separates the chamber into a reservoir and an operating chamber, with the input coupling member being disposed in the operating chamber.
In conventional fluid coupling devices ("fan drives") which are used to drive radiator cooling fans on vehicles, the valve plate includes a valving arrangement operable in response to changes in ambient air temperature to permit fluid to flow from the reservoir into the operating chamber. Typical fan drives include a discharge port defined near the outer periphery of the operating chamber with some form of pumping element, such that a small quantity of fluid is continually pumped from the operating chamber back to the reservoir chamber during normal operation.
There is the potential for the temperature of the fluid in the fan drive to exceed a predetermined maximum temperature. The potential for this to occur increases as the speed differential ("slip speed") increases between the input coupling member and the output coupling member. In other words, excessive shearing of the fluid can cause the fluid to exceed the predetermined maximum temperature. When this occurs, the fluid (typically a silicone fluid) undergoes a process in which the fluid first "droops", i.e., there is a physical breakdown of the polymer chains such that the fluid viscosity decreases, and the torque transmitting capability of the coupling decreases substantially. Then, with continued excess fluid temperature, the fluid again begins to cross-link, but does so excessively, and eventually the fluid "gels" or becomes nearly solid. If the fluid reaches this latter condition, the coupling device operates as if it were solid, with little or no slip speed in the engaged mode, and no capability of operating in the disengaged mode.
One attempt to overcome the above-described problem is illustrated in U.S. Pat. No. 5,248,018, assigned to the assignee of the present invention and incorporated herein by reference. In the device of the cited patent, the valve arm is generally Y-shaped, having two fill port covering portions, the first of which covers the fill port at low ambient temperatures. As ambient temperature increases, the valve arm rotates to a position in which the valve covering portions of the valve arm are on either side of the fill port, thus permitting filling. As internal fluid temperature increases as a result of high slip speeds, heat is dissipated from the fluid through the cover, further increasing the temperature of the ambient air around the external bimetal coil. As a result, the valve arm continues to rotate in the same direction until the fill port is covered by the second port covering portion. Once the fill port is covered, and the fluid in the operating chamber is pumped back to the reservoir, the fluid coupling operates in the disengaged mode, with the fluid thus being protected from the overheating condition described above.
Unfortunately, it has been found that the arrangement of the cited patent is most likely to be satisfactory on fan drives having relatively thin, stamped steel covers, as is typically the case on the relatively smaller, automotive fan drives. However, on the relatively larger, higher torque fan drives typically utilized on light trucks, and having cast aluminum covers, the heat transfer from the fluid to the external bimetal coil is generally insufficient to heat the bimetal coil enough to achieve the desired protection of the viscous fluid.